During a conversation over the phone or the like, sound recognition, sound recording, and the like, it is preferable to collect only a target sound (a speaker's voice). However, in an environment in which a sound input device for sound collection is used, a sound other than the target sound, such as a background noise, may exist. Thus, sound input devices having a function that allows a target sound to be accurately extracted even when the devices are used in an environment in which a noise exists, that is, a function of removing a noise, are under development. In addition, recently, electronic equipment has been downsized, and the sound input devices have also been downsized.
Concerning the sound input devices (hereinafter also referred to as “microphones”), for example, Japanese Patent Laying-Open No. 06-339192 (Patent Document 1) discloses a microphone unit for “allowing variations in individual acoustic impedance characteristics to be readily adjusted to a predetermined characteristic in an omnidirectional electric capacitor microphone” ([Object] in [Abstract]). The microphone unit is described as “In a microphone unit 1 in which a diaphragm 5, a back plate 6, a cover plate 9, and the like are housed in a unit case 2, a path 12 connecting a rear portion of back plate 6 with an outside is formed, and path 12 is provided with an adjustment mechanism changing an acoustic impedance of path 12. As an example of the adjustment mechanism, path 12 is provided to cover plate 9, and a bolt 14 is screwed into path 12. The acoustic impedance of path 12 is changed by changing the degree with which bolt 14 is screwed” ([Means for Solution] in [Abstract]).
In addition, Japanese Patent Laying-Open No. 08-098289 (Patent Document 2) discloses a “ground-based microphone having flat sensitivity frequency characteristics by improving sensitivity frequency characteristics in a high frequency range” ([Object] in [Abstract]). The ground-based microphone is described as “In a container 1a of a microphone element 1, an extremely small pressure equivalent sound hole 1b connecting an inside and an outside of container 1a inside a diaphragm 1c is provided” ([Means for Solution] in [Abstract]).
Japanese Patent Laying-Open No. 2007-060228 (Patent Document 3) discloses a silicon microphone package “having a cavity and a vent hole (leak hole) that allow the package to obtain excellent sound receiving characteristics, and capable of being fabricated at a low cost” ([Object] in [Abstract]). It is described that “A silicon microphone package 1 includes a printed circuit board 4 in which a silicon microphone chip 2 is mounted over an opening 4a, a casing 5 placed on printed circuit board 4 to surround silicon microphone chip 2, and an insulating plate 6 placed on a rear surface of casing 5. In the rear surface of casing 5, a groove 5a and an opening 5b toward which groove 5a faces are provided. Insulating plate 6 is provided with a through hole 6a. By placing casing 5 on printed circuit board 4, a space surrounded by casing 5 and printed circuit board 4 serves as a cavity 7, and by placing insulating plate 6 on the rear surface of casing 5, through hole 6a, groove 5a, and opening 5b serve as a vent hole 8 connecting cavity 7 with an external space” ([Means for Solution] in [Abstract]).
Japanese Patent Laying-Open No. 2001-054195 (Patent Document 4) discloses an “electret capacitor microphone having a small size and less number of components that prevents an adverse effect due to a reflected wave during vibration” ([Object] in [Abstract]). The electret capacitor microphone is described as “In an electret capacitor microphone in which a fixed electrode 6 and a vibrating membrane 4 are arranged at a certain interval to detect a sound signal given to vibrating membrane 4 based on a change in capacitance therebetween, fixed electrode 6, vibrating membrane 4, and a semiconductor element 8 are housed in a concave portion 9a of a box-shaped case 9 made of an insulating material, and a rear chamber 12 is provided in a wall surface of concave portion 9a” ([Means for Solution] in [Abstract]).
Japanese Patent Laying-Open No. 2003-163996 (Patent Document 5) discloses an “electret capacitor microphone having a small size and less number of components that prevents an adverse effect due to a reflected wave during vibration” ([Problem] in [Abstract]). The electret capacitor microphone is described as “In an electret capacitor microphone in which a fixed electrode 6 and a vibrating membrane 4 are arranged at a certain interval to detect a sound signal given to vibrating membrane 4 based on a change in capacitance therebetween, fixed electrode 6, vibrating membrane 4, and a semiconductor element 8 are housed in a concave portion 9a of a box-shaped case 9 made of an insulating material, and a rear chamber 12 is provided in a wall surface of concave portion 9a” ([Means for Solution] in [Abstract]).